Bump manufacturing method

ABSTRACT

A method of forming bumps on the active surface of a silicon wafer. A first under-ball metallic layer is formed over the active surface of the wafer. A second under-ball metallic layer is formed over the first under-ball metallic layer. A portion of the second under-ball metallic layer is removed to expose the first under-ball metallic layer. A plurality of solder blocks is implanted over the second under-ball metallic layer. A reflux operation is conducted and then the exposed first under-ball metallic layer is removed so that only the first under-ball metallic layer underneath the second under-ball metallic layer remains.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of Taiwan applicationserial no. 91102775, filed Feb. 19, 2002.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention relates to a method of manufacturing bumps. Moreparticularly, the present invention relates to a process of fabricatingbumps that require a shorter contact period with etchant and a thinnerphotoresist layer.

2. Description of Related Art

In this information explosion age, electronic products are used almosteverywhere. Computer and processing stations driven by powerfulintegrated circuits are employed in offices, educational institutions,recreational industries, business and commercial companies. Aselectronic technology continues to progress, products having morepowerful functions and more attuned to personal needs are developed.Furthermore, most electronic products are increasingly light and compactthanks to the efficient fabrication of many types of high-densitysemiconductor packages. A major innovation is the flip chip designcapable of cramming a considerable number of integrated circuitstogether. In a flip-chip design, a plurality of bumps is formed on thebonding pads of a silicon chip. Each bump directly contacts with acorresponding contact point on a substrate so that the chip and thesubstrate are electrically connected. Compared with the conventionalwire-bonding and tape automated bonding (TAB) method of joining a chipwith a substrate, the flip-chip design has a shorter overall conductivepath and hence a better electrical connectivity. In addition, thebackside of the chip may be exposed to facilitate heat dissipationduring operation. Due to the distinguishing advantages of flip-chippackages, semiconductor manufacturing favors its production.

FIGS. 1 to 7 are partially magnified cross-sectional views of structureson the surface of a silicon wafer showing the progression of steps forproducing bumps on the wafer according to a conventional method. Asshown in FIG. 1, a silicon wafer 110 is provided. The wafer 110 has anactive surface 112. The wafer 110 further includes a passivation layer114 and a plurality of bonding pads 116 (only one of them is shown) onthe active surface 112 of the wafer 110. The passivation layer 114exposes the bonding pad 116.

As shown in FIG. 2, an adhesion layer 120 is formed over the activesurface 112 of the wafer 110 by conducting a sputtering operation. Theadhesion layer 120 covers the bonding pad 116 and the passivation layer114. Thereafter, a barrier layer 130 is formed over the adhesion layer120 by conducting a sputtering or an electroplating operation. Awettable layer 140 is formed over the barrier layer 130 by conducting asputtering or an electroplating operation. Here, the fabrication of aso-called under-ball metallic layer 142 is complete. The under-ballmetallic layer 142 actually is a composite layer comprising the adhesionlayer 120, the barrier layer 130 and the wettable layer 140.

As shown in FIG. 3, a photolithographic operation is conducted byforming a photoresist layer 150 over the wettable layer 140, exposingthe photoresist layer 150 to light and then developing the photoresistlayer. Ultimately, a pattern (not shown) is transferred to thephotoresist layer 150. The photoresist layer 150 now contains aplurality of openings 152 (only one is shown) that exposes the wettablelayer 140 above the bonding pad 116.

As shown in FIG. 4, metal is deposited to refill the opening byconducting an electroplating operation so that a plurality of solderblocks 160 (only one is shown) is formed inside the opening 152 of thephotoresist layer 150. The solder block 160 completely covers theexposed wettable layer 140.

As shown in FIGS. 4 and 5, the photoresist layer 150 is completelyremoved from the top of the wettable layer 140.

As shown in FIGS. 5 and 6, the under-ball metallic layer 142 outside thesolder block 160 region is removed by etching. Consequently, only theresidual under-ball metallic layer 142 remains underneath the solderblock 160. The passivation layer 114 above the wafer 110 is now exposed.

As shown in FIG. 7, a reflux operation is conducted by sprinkling fluxover the wafer 100 and heating to a temperature such that the solderblock 160 starts to melt and turns into a hemispherical shape bump 170.The bump 170 is actually a composite structure that includes theunder-ball metallic layer 142 and the solder block 160.

In the fabrication process as shown in FIGS. 1 to 7, etchant is used toremove the wettable layer 140, the barrier layer 130 and the adhesionlayer 120 in sequence (not shown). During etching, the etchant may comein contact with the solder block 160 and etch away a portion of thesolder block 160 layer. Hence, overall thickness of the solder block 160may be reduced leading to material wastage and difficulty in controllingsolder block 160 quality. Furthermore, when the etchant for etching thewettable layer 140 and the barrier layer 130 is improperly prepared, theetchant may act on the solder block 160. The etchant may peel off thesolder block 160 from the wettable layer 140 before the wettable layer140 and the barrier layer 130 are removed. Moreover, to match thedimension of the under-ball metallic layer 160, cross-sectional area ofthe opening 152 in the photoresist layer 150 must be set to a smallvalue so that the solder block 160 inside the opening 152 is thick.Consequently, the photoresist layer 150 must have comparable thicknessresulting in a higher cost of production.

SUMMARY OF INVENTION

Accordingly, one object of the present invention is to provide a processof fabricating bumps capable of reducing contact with etchant and thewasting of solder blocks so that the solder block is more accuratelyshaped.

A second object of this invention is to provide a process of fabricatingbumps such that the peeling of solder blocks due to etchant isprevented.

A third object of this invention is to provide a process of fabricatingbumps that involves the formation of an opening having a largecross-sectional area in a photoresist layer during the photolithographicprocess. Consequently, a smaller amount of metallic material needs to bedeposited into the opening and the resulting solder block has a minimalheight. Ultimately, a thinner photoresist layer is required and henceproduction cost is reduced.

Note in the following description that the use of the preposition “over”as in “a second layer is formed over a first layer” means that thesecond layer is either in contact with the first layer or simply abovethe first layer.

To achieve these and other advantages and in accordance with the purposeof the invention, as embodied and broadly described herein, theinvention provides a process of fabricating bumps on a silicon wafer.The wafer has an active surface with a passivation layer and a pluralityof bonding pads thereon. The passivation layer exposes the bonding pads.First, an adhesion layer is formed over the active surface of the wafer.The adhesion layer covers the bonding pads and the passivation layer. Abarrier layer is formed over the adhesion layer and then a wettablelayer is formed over the barrier layer.

A first photolithographic process is carried out to form a plurality ofphotoresist blocks over the wettable layer. Thereafter, a first etchingoperation is conducted to remove the wettable layer and the barrierlayer outside the photoresist covered region. The photoresist blocks areremoved.

A second photolithographic process is carried out to form a photoresistlayer over the adhesion layer. The photoresist layer has a plurality ofopenings that expose the wettable layer and the adhesion layer aroundthe barrier layer. A metal-filling operation is conducted to form solderblocks inside the openings in the photoresist layer. The solder blockscover the wettable layer and the adhesion layer around the barrierlayer. The photoresist layer is removed.

A first reflux operation is carried out so that the solder block changesto a blob having a hemispherical profile and the solder block alsoretracts into the upper surface of the wettable layer without extendinginto the adhesion layer.

A second etching operation is carried out so that the exposed adhesionlayer is removed while the adhesion layer underneath the barrier layeris retained. In the meantime, the passivation layer over the wafer isexposed. Finally, a second reflux operation is conducted.

According to one preferred embodiment of this invention, the secondreflux operation is a selective process. In addition, the first refluxoperation may be carried out before the step of removing the photoresistlayer. Furthermore, the adhesion layer can be a titanium, a titaniumtungsten alloy, aluminum or chromium layer, the barrier layer can be anickel-vanadium alloy layer and the wettable layer can be a copper,palladium or gold layer.

In brief, a two-stage process is used to etch the under-ball metalliclayer according to this invention. In the first state, the wettablelayer and the barrier layer are etched. Since the solder blocks are notformed over the wettable layer, etchant will not attack the solderblock. Etchant will contact the solder block only when the adhesionlayer is etched in the second etching operation. Hence, the bumpfabrication process is able to minimize volume reduction of the solderblocks due to etchant contact. Consequently, the solder blocks can havea more precise dimension.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

FIGS. 1 to 7 are partially magnified cross-sectional views of structureson the surface of a silicon wafer showing the progression of steps forproducing bumps on the wafer according to a conventional method;

FIGS. 8 to 17 are partially magnified cross-sectional views ofstructures on the surface of a silicon wafer showing the progression ofsteps for forming bumps over a silicon wafer according to a firstpreferred embodiment of this invention; and

FIGS. 18 to 21 are partially magnified cross-sectional views ofstructures on the surface of a silicon wafer showing the progression ofsteps for forming bumps over a silicon wafer according to a secondpreferred embodiment of this invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIGS. 8 to 17 are partially magnified cross-sectional views ofstructures on the surface of a silicon wafer showing the progression ofsteps for forming bumps over a silicon wafer according to a firstpreferred embodiment of this invention. As shown in FIG. 8, a siliconwafer 310 is provided. The wafer 310 has an active surface 312 with apassivation layer 314 and a plurality of bonding pads 316 (only one isshown) thereon. The passivation layer 314 exposes the bonding pads 316.

As shown in FIG. 9, an adhesion layer 320 is formed over the activesurface 312 of the wafer 310 by sputtering or evaporation plating. Theadhesion layer 320 covers both the bonding pads 316 and the passivationlayer 314. The adhesion layer 320 can be made from a material including,for example, titanium, titanium-tungsten alloy, aluminum or chromium. Abarrier layer 330 is formed over the adhesion layer 320 by sputtering,electroplating or evaporation plating. The barrier layer 330 can be madefrom a material such as nickel-vanadium alloy, for example. A wettablelayer 340 is formed over the barrier layer 330 by sputtering,electroplating or evaporation plating. The wettable layer 340 can bemade from a material including, for example, copper, palladium or gold.Hence, a so-called under-ball metallic layer 342 that comprises theadhesion layer 320, the barrier layer 330 and the wettable layer 340 isthereby formed.

As shown in FIG. 10, a first photolithographic process is carried out byforming a photoresist layer over the wettable layer 340, exposing thephotoresist layer through a mask and developing the photoresist layerchemically. Ultimately, a pattern (not shown) is transferred from themask to the photoresist layer. In other words, a plurality ofphotoresist blocks 350 (only one is shown) is formed in the locationsfor forming the bumps directly above the bonding pads 316.

As shown in FIG. 11, a first etching operation is conducted to removethe wettable layer 340 and the barrier layer 330 outside the photoresistblocks 350 so that residual wettable layer and barrier layer remainunderneath the photoresist blocks 350. The copper wettable layer 340 isetched using an etchant containing ammonium hydroxide and hydrogenperoxide having a composition according to U.S. Pat. No. 6,222,279, oran etchant containing potassium sulfate (K₂SO₄) and glycerol accordingto U.S. Pat. No. 5,486,282 and U.S. Pat. No. 5,937,320 or some otherknown chemical etchants. The nickel-vanadium barrier layer 330 is etchedusing sulfuric acid (H₂SO₄) as the etchant. The actual etching operationdiffers according to the actual working environment and is brieflydescribed in the following.

In a first embodiment, the barrier layer 330 is etched at roomtemperature using 1%˜98% sulfuric acid (H₂SO₄). When the barrier layer330 has a thickness between 2000 Å to 4000 Å, an etching periodexceeding 2 hours is required.

In a second embodiment, the barrier layer 330 is etched at a temperatureabove 80° C. using 1%˜98% sulfuric acid (H₂SO₄). When the barrier layer330 has a thickness between 2000 Å to 4000 Å, an etching periodexceeding 2 hours is required.

In a third embodiment, the barrier layer 330 is etched in anelectrochemical etching operation. For example, a current density ofabout 0.001˜0.02 A/cm², preferably 0.0025 A/cm², is passed while theetching is conducted at room temperature using 10% sulfuric acid(H₂SO₄). When the barrier layer 330 has a thickness between 2000 Å to4000 Å, an etching period between 20 seconds to 110 seconds is required.However, the optimal etching period is between 20 seconds to 40 seconds.In addition, either a constant current or a pulse current may be appliedduring etching.

Furthermore, the nickel-vanadium barrier layer 330 may be etched using adiluted phosphoric acid solution having a composition according to U.S.Pat. No. 5,508,229.

In all the aforementioned etching processes, the active surface of thewafer is usually cleaned using de-ionized water so that any residualetchant on the bumps and active surface from a previous etchingoperation is removed. This ensures a higher yield after the completionof the bump manufacturing process.

As shown in FIG. 12, the photoresist blocks 350 are removed.

As shown in FIG. 13, a second photolithographic process is carried outto form a photoresist layer 360 over the adhesion layer 320 and thewettable layer 340. Through photo-exposure and photoresist development,a pattern (not shown) is transferred from a mask to the photoresistlayer 360. The photoresist layer 360 has a plurality of openings 362(only one is shown) that exposes the residual wettable layer 340 on thebonding pads 316 and the adhesion layer 320 around the residual barrierlayer 330.

As shown in FIG. 14, metallic material is deposited into the openings362 in the photoresist layer 360 by electroplating to form a pluralityof solder blocks 370 (only one is shown). The solder blocks 370 coverthe wettable layer 340 and the adhesion layer 320 around the barrierlayer 330. Thereafter, the photoresist layer 360 is removed from theupper surface of the adhesion layer 320 to form a structure shown inFIG. 15.

As shown in FIG. 16, a first reflux operation is carried. Flux materialis sprinkled onto the wafer and the wafer is heated until the solderblocks 370 partially melt. Through the heating, the solder blocks 370are transformed into a blob of material having a hemispherical profile.Note that material constituting the solder blocks 370 must not wet theadhesion layer 320 so that the solder blocks 370 can retract onto theupper surface of the wettable layer 340 without extending to theadhesion layer 320. Next, a second etching operation is carried out toremove the exposed adhesion layer 320 so that only the residual adhesionlayer 320 underneath the barrier layer 330 remains. In the meantime, thepassivation layer 314 covering the wafer 310 is exposed to form astructure as shown in FIG. 17. If the adhesion layer 320 is atitanium-tungsten alloy layer, etchant containing hydrogen peroxide(H₂O₂), ethylene diamine tetraacetic (EDTA) and potassium sulfate(K₂SO₄) and having a composition according to U.S. Pat. No. 5,462,638can be used so that the etching effect on the solder blocks 360 isminimal. If the adhesion layer is a chromium layer, an etchantcontaining hydrochloric acid (HCl) having a composition according toU.S. Pat. No. 5,162,257 can be used so that the etching effect on thesolder blocks 360 is also minimal. If the adhesion layer is a titaniumlayer, an etchant containing ammonium hydroxide and hydrogen peroxide(H₂O₂) having a composition according to U.S. Pat. No. 5,162,257 can beused so that the etching effect on the solder blocks 360 is alsominimal. Alternatively, hydrogen fluoride (HF) can be used as an etchantfor etching titanium adhesion layer 320. If the adhesion layer is analuminum layer, an etchant containing phosphoric acid and acetic acidhaving a composition according to U.S. Pat. No. 5,508,229 can be used.However, during the second etching operation, the upper surface of thesolder blocks 370 is also etched leading to a highly irregular surface.Hence, a second reflux operation may be introduced by selection. In thesecond reflux operation, flux material is sprinkled onto the wafer 310and the wafer 310 is heated until the solder blocks 370 is planarized.This finishes the fabrication of the bumps 380. Note that each bump is acomposite structure comprising an under-ball metallic layer 342 and asolder block 370. The wafer 310 is finally sliced into a plurality ofchips.

In the first embodiment, the wafer 310 may be sliced into chipsimmediately after the second etching operation. The second refluxoperation is an optional step that can be added on demand.

In the fabrication process with reference to FIGS. 8 to 17, a two-stageetching process is used to etch the under-ball metallic layer 342.During the first etching operation, that is, the etching of the wettablelayer 340 and the barrier layer 330, etchant will not etch the solderblocks 370 because the solder blocks 370 are yet to be formed over thewettable layer 340. Etchant will contact the solder blocks 370 only whenthe adhesion layer 320 is etched in the second etching operation.Consequently, the period of contact of solder blocks 370 with etchant isreduced and hence dimensional reduction of the solder blocks 370 isminimized. Moreover, peeling of the solder blocks due to contact withetchant during fabrication is also prevented. Furthermore, the openings362 in the photoresist layer 360 can have a larger cross-sectional area.Hence, the solder blocks 370 have a lower thickness for the same volume.Thus, a thinner photoresist layer 360 is required resulting in a lowerproduction cost. In addition, cross-sectional profile of the openings362 in the photoresist layer 360 can have a variety of shapes includinga circular or octagonal shape.

In the aforementioned fabrication process, the reflux operation iscarried out after the photoresist layer is removed. However, the refluxoperation may also be carried out before photoresist removal as shown inFIGS. 18 to 21. FIGS. 18 to 21 are partially magnified cross-sectionalviews of structures on the surface of a silicon wafer showing theprogression of steps for forming bumps over a silicon wafer according toa second preferred embodiment of this invention. In the secondembodiment, only the steps that differ from the first embodiment aredescribed.

As shown in FIG. 18, a metal-filling operation is conducted after thesecond photolithographic process. In the metal-filling operation,metallic material is deposited into the openings 662 in the photoresistlayer 660 to form a plurality of solder blocks (only one is shown) byelectroplating. The solder blocks 670 cover the wettable layer 640 andthe adhesion layer 620 around the barrier layer 630.

As shown in FIG. 19, a first reflux operation is conducted and then thewafer 610 is heated until the solder blocks 670 partially melt andtransform into a blob of material having a hemispherical profile. Inthis invention, a solder material that does not wet the adhesion layer620 is selected to form the solder blocks 670. Hence, the solder blocks670 can retract onto the upper surface of the wettable layer 640 withoutextending to the adhesion layer 620. Thereafter, the photoresist layer660 is removed from the upper surface of the adhesion layer 620 to forma structure as shown in FIG. 20. A second etching operation is carriedout to remove the exposed adhesion layer 620 so that only a residualadhesion layer 620 remains underneath the barrier layer 630. Thepassivation layer 614 on the wafer 610 is also exposed to form astructure as shown in FIG. 21. Since the solder blocks 670 are likelyetched due to contact with etchant during the second etching operation,the upper surface of the solder blocks 670 has a roughened surface. Toplanarize the solder blocks 670, a second reflux operation may becarried out on demand. This finishes the fabrication of the bumps 680.Note that each bump is a composite structure comprising an under-ballmetallic layer 642 and a solder block 670. The wafer 610 is finallysliced into a plurality of chips.

In this invention, the reflux operation is carried out before theetching process. Hence, the deposition of metallic material to formsolder blocks is not limited to electroplating. Other methods includingnet printing, ball implant or directly filling the photoresist openingusing a scrapper are possible.

Material constituting the under-ball metallic layer is also not limitedto the aforementioned. Various other types of under-ball metallicmaterials may similarly be applied to the fabrication of bumps as longas solder block material does not wet the adhesion layer. The solderblocks can be made from a material such as gold, tin-lead alloy orlead-free metal while the bonding pads can be made from a material suchas aluminum or copper.

The under-ball metallic layer according to this invention need not belimited to just three layers (the adhesion layer, the barrier layer andthe wettable layer). Other numbers of conductive layers is possible. Forexample, the under-ball metallic layer can be a structure with fourlayers, including a chromium layer, a chromium-copper alloy layer, acopper layer and a silver layer. Alternatively, the under-ball metalliclayer can be a structure with two layers, including a lower layer suchas a titanium-tungsten alloy layer or a titanium layer and an upperlayer such as a copper layer, a nickel layer or a gold layer.

Although the bumps are directly formed on the active surface of asilicon wafer in the aforementioned embodiments, the bumps may also formelsewhere. For example, the bumps may form over a redistribution layerafter the redistribution layer is formed on a silicon wafer.

In conclusion, major advantages of this invention includes:

1. A two-stage etching process is used to etch the under-ball metalliclayer. During the first etching operation, that is, the etching of thewettable layer and the barrier layer, etchant will not etch the solderblocks because the solder blocks are yet to be formed over the wettablelayer. Etchant will contact the solder blocks only when the adhesionlayer is etched in the second etching operation. Consequently, theperiod of contact of solder blocks with etchant is reduced and hencedimensional reduction of the solder blocks is minimized.

2. Peeling of the solder blocks due to contact with etchant duringfabrication is also prevented.

3. The openings in the photoresist layer can have a largercross-sectional area. Hence, the solder blocks have a lower thicknessfor the same volume. Thus, a thinner photoresist layer is requiredresulting in a lower production cost.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A method or forming bumps on a silicon waferhaving an active surface with a passivation layer and a plurality ofbonding pads thereon such that the passivation layer exposes the bandingpads, the method comprising the steps of: forming an adhesion layer overthe active surface of the wafer, the adhesion layer covering both thebonding pads and the passivation layer, forming a barrier layer over theadhesion layer; forming a wettable layer over the barrier layer;conducting a first photolithography process to from a plurality ofphotoresist blocks on the wettable layer; conducting a first etchingoperation to remove the wettable layer and the barrier layer so thatonly the residual wettable layer and barrier layer underneath thephotoresist blocks remain; after conducting the first etching operation,removing the photoresist blocks; after removing the photoresist blocks,conducting a second photolithography process to form a photoresist layerover the adhesion layer, wherein the photoresist layer has a pluralityof openings that expose the wettable layer and the adhesion layer aroundthe barrier layer; after conducting the second photolithography process,conducting a metal-filling operation to farm a solder material insidethe openings of the photoresist layer, wherein the solder materialcovers the wettable layer and the adhesion layer around the barrierlayer; after conducting the metal-filling operation, removing thephotoresist layer; after removing the photoresist layer, conducting afirst reflow operation to transform the solder material into a pluralityof solder balls having a hemispherical profile, and the solder ballsretracting onto the upper surface of the wettable layer withoutextending onto the adhesion layer; after conducting the first reflowoperation, conducting a second etching operation to remove a portion ofthe adhesion layer so that only residual adhesion layer underneath thebarrier layer is retained and the passivation layer on the wafer isexposed to the outside; and conducting a second reflow operation.
 2. Themethod of claim 1, wherein material constituting the adhesion layer isselected from a group consisting of titanium, titanium-tungsten alloy,aluminum and chromium.
 3. The method of claim 2, wherein etchant foretching the adhesion layer in the second etching operation containshydrogen peroxide (H₂O₂), ethylene diaminete traacetic (EDTA) andpotassium sulfate (K₂SO₄) when the adhesion layer is a titanium-tungstenalloy layer.
 4. The method of claim 2, wherein etchant for etching theadhesion layer in the second etching operation contains hydrogenchloride (HCl) when the adhesion layer is a chromium layer.
 5. Themethod of claim 2, wherein etchant for etching the adhesion layer in thesecond etching operation contains ammonium hydroxide (NH₄OH) andhydrogen peroxide (H₂O₂) when the adhesion layer is a titanium layer. 6.The method of claim 2, wherein etchant for etching the adhesion layer inthe second etching operation contains hydrogen fluoride (HF) when theadhesion layer is a titanium layer.
 7. The method of claim 2, whereinetchant for etching the adhesion layer in the second etching operationcontains phosphoric acid and acetic acid when the adhesion layer is analuminum layer.
 8. The method of claim 1, wherein material constitutingthe barrier layer includes nickel-vanadium alloy.
 9. The method of claim8, wherein the etchant for etching the barrier layer in the firstetching operation contains sulfuric acid.
 10. The method of claim 9,wherein the barrier layer having a thickness between 2000 Å and 4000 Åis etched for over 2 hours at room temperature using a sulfuric acidetchant having a concentration between 1%-98%.
 11. The method of claim9, wherein the barrier layer having a thickness between 2000 Å and 4000Å is etched for over 2 hours at 80° C. using a sulfuric acid etchanthaving a concentration between 1%-98%.
 12. The method of claim 9,wherein the barrier layer having a thickness between 2000 Å and 4000 Åis etched in the first etching operation by conducting anelectrochemical etching operation at room temperature for 20 to 110seconds using a current density between 0.001˜0.02 A/cm² and sulfuricacid at 10% concentration.
 13. The method of claim 8, wherein thebarrier layer is etched using diluted phosphoric acid in the firstetching operation.
 14. The method of claim 1, wherein materialconstituting the wettable layer is selected from a group consisting ofcopper, palladium and gold.
 15. The method of claim 14, wherein thewettable layer is etched in the first etching operation using an etchantcontaining ammonium hydroxide and hydrogen peroxide if the wettablelayer is a copper layer.
 16. The method of claim 14, wherein thewettable layer is etched in the first etching operation using an etchantcontaining potassium sulfate (K₂SO₄) and glycerol if the wettable layeris a copper layer.
 17. The method of claim 1, wherein the soldermaterial does not wet the adhesion layer.
 18. A method of forming bumpson a silicon wafer having an active surface with a passivation layer anda plurality of bonding pads thereon such that the passivation layerexposes the bonding pads, the method comprising the steps of: forming anadhesion layer over the active surface of the wafer, the adhesion layercovering both the bonding pads and the passivation layer; forming abarrier layer over the adhesion layer; forming a wettable layer over thebarrier layer; conducting a first photolithography process to from aplurality of photoresist blocks on the wettable layer, conducting afirst etching operation to remove the wettable layer and the barrierlayer so that only the residual wettable layer and barrier layerunderneath the photoresist blocks remain; after conducting the firstetching operation, removing the photoresist blocks; after removing thephotoresist blocks, conducting a second photolithography process to forma photoresist layer over the adhesion layer, wherein the photoresistlayer has a plurality of openings that expose the wettable layer and theadhesion layer around the barrier layer; after conducting the secondphotolithography process, conducting a metal-filling operation to form asolder material inside the openings of the photoresist layer, whereinthe solder material covers the wettable layer and the adhesion layeraround the barrier layer; after conducting the metal-filling operation,conducting a first reflow operation to transform the solder materialinto a plurality of solder balls having a hemispherical profile, and thesolder balls retracting onto the upper surface of the wettable layerwithout extending onto the adhesion layer; after conducting the firstreflow operation, removing the photoresist layer; after removing thephotoresist layer, conducting a second etching operation to remove aportion of the adhesion layer so that only residual adhesion layerunderneath the barrier layer is retained and the passivation layer onthe wafer is exposed to the outside; and conducting a second reflowoperation.
 19. The method of claim 1, wherein material constituting theadhesion layer is selected from a group consisting of titanium,titanium-tungsten alloy, aluminum and chromium.
 20. The method of claim19, wherein etchant for etching the adhesion layer in the second etchingoperation contains hydrogen peroxide (H₂O₂), ethylene diaminetetraacetic (EDTA) and potassium sulfate (K₂SO₄) when the adhesion layeris a titanium-tungsten alloy layer.
 21. The method of claim 19, whereinetchant for etching the adhesion layer in the second etching operationcontains hydrogen chloride (HCl) when the adhesion layer is a chromiumlayer.
 22. The method of claim 19, wherein etchant for etching theadhesion layer in the second etching operation contains ammoniumhydroxide (NH₄OH) and hydrogen peroxide (H₂O₂) when the adhesion layeris a titanium layer.
 23. The method of claim 19, wherein etchant foretching the adhesion layer in the second etching operation containshydrogen fluoride (HF) when the adhesion layer is a titanium layer. 24.The method of claim 19, wherein etchant for etching the adhesion layerin the second etching operation contains phosphoric acid and acetic acidwhen the adhesion layer is an aluminum layer.
 25. The method of claim18, wherein material constituting the barrier layer includesnickel-vanadium alloy.
 26. The method of claim 25, wherein the etchantfor etching the barrier layer in the first etching operation containssulfuric acid.
 27. The method of claim 26, wherein the barrier layerhaving a thickness between 2000 Å and 4000 Å is etched for over 2 hoursat room temperature using a sulfuric acid etchant having a concentrationbetween 1%˜98%.
 28. The method of claim 26, wherein the barrier layerhaving a thickness between 2000 Å and 4000 Å is etched for over 2 hoursat 80° C. using a sulfuric acid etchant having a concentration between1%˜98%.
 29. The method of claim 26, wherein the barrier layer having athickness between 2000 Å and 4000 Å is etched in the first etchingoperation by conducting an electrochemical etching operation at roomtemperature for 20 to 110 seconds using a current density between0.001˜0.02 A/cm² and sulfuric acid at 10% concentration.
 30. The methodof claim 25, wherein the barrier layer is etched using dilutedphosphoric acid in the first etching operation.
 31. The method of claim18, wherein material constituting the wettable layer is selected from agroup consisting of copper, palladium and gold.
 32. The method of claim31, wherein the wettable layer is etched in the first etching operationusing an etchant containing ammonium hydroxide and hydrogen peroxide ifthe wettable layer is a copper layer.
 33. The method of claim 31,wherein the wettable layer is etched in the first etching operationusing an etchant containing potassium sulfate (K₂SO₄) and glycerol ifthe wettable layer is a copper layer.
 34. The method of claim 18,wherein material constituting the solder blocks does not wet theadhesive layer.
 35. A method of forming bumps over a wafer having anactive surface thereon, the method comprising the steps of: forming afirst under-ball metallic layer over the active surface of the wafer;forming a second under-ball metallic layer over the first under-ballmetallic layer, conducting a first photolithography process to form aplurality of photoresist blocks over the second under-ball metalliclayer; conducting a first etching operation to remove the secondunder-ball metallic layer so that only the second under-ball metalliclayer underneath the photoresist blocks remains; after conducting thefirst etching operation, removing the photoresist blocks; after removingthe photoresist blocks, conducting a second photolithography process toform a photoresist layer over the first under-ball layer, wherein thephotoresist layer has a plurality of openings that expose the secondunder-ball metallic layer; after conducting the second photolithographyprocess, conducting a metal-filling operation to fill a solder materialinto the openings of the photoresist layer, the solder material coveringthe second under-ball metallic layer, after conducting the metal-fillingoperation, removing the photoresist layer; after removing thephotoresist layer, conducting a first reflow operation to transform thesolder material into a plurality of solder balls; and after conductingthe first reflow operation, conducting a second etching operation toremove a portion of the first under-ball metallic layer so that only thefirst under-ball metallic layer underneath the second under-ballmetallic layer remains.
 36. The method of claim 35, wherein afterconducting the second etching operation, a second reflow operation isconducted.
 37. The method of claim 35, wherein the step of forming thesecond under-ball metallic layer over the first under-ball metalliclayer includes the sub-steps of: forming a barrier layer over the firstunder-ball metallic layer; and forming a wettable layer over the barrierlayer.
 38. The method of claim 37, wherein material constituting thebarrier layer includes nickel-vanadium alloy.
 39. The method of claim38, wherein the first etching operation is carried out using an etchantcontaining sulfuric acid.
 40. The method of claim 39, wherein thebarrier layer having a thickness ranging from 2000 Å to 4000 Å is etchedfor over 2 hours at room temperature using a sulfuric acid etchanthaving a concentration between 1%˜98%.
 41. The method of claim 39,wherein the barrier layer having a thickness ranging from 2000 Å to 4000Å is etched for over 2 hours at 80° C. using a sulfuric acid etchanthaving a concentration between 1%˜98%.
 42. The method of claim 39,wherein the barrier layer having a thickness ranging from 2000 Å to 4000Å is etched in the first etching operation by conducting anelectrochemical etching operation at room temperature for 20 to 110seconds using a current density between 0.001˜0.02 A/cm² and sulfuricacid at 10% concentration.
 43. The method of claim 38, wherein thebarrier layer is etched using diluted phosphoric acid in the firstetching operation.
 44. The method of claim 37, wherein materialconstituting the wettable layer is selected from a group consisting ofcopper, palladium and gold.
 45. The method of claim 44, wherein thewettable layer is etched in the first etching operation using an etchantcontaining ammonium hydroxide and hydrogen peroxide if the wettablelayer is a copper layer.
 46. The method of claim 44, wherein thewettable layer is etched in the first etching operation using an etchantcontaining potassium sulfate (K₂SO₄) and glycerol if the wettable layeris a copper layer.
 47. The method of claim 35, wherein the firstunder-ball metallic layer includes an adhesion layer fabricated using amaterial selected from a group consisting of titanium, titanium-tungstenalloy, aluminum and chromium.
 48. The method of claim 47, whereinetchant for etching the adhesion layer in the second etching operationcontains hydrogen peroxide (H₂O₂), ethylene diamino tetraacetic (EDTA)and potassium sulfate (K₂SO₄) when the adhesion layer is atitanium-tungsten alloy layer.
 49. The method of claim 47, whereinetchant for etching the adhesion layer in the second etching operationcontains hydrogen chloride (HCl) when the adhesion layer is a chromiumlayer.
 50. The method of claim 47, wherein etchant for etching theadhesion layer in the second etching operation contains ammoniumhydroxide (NH₄OH) and hydrogen peroxide (H₂O₂) when the adhesion layeris a titanium layer.
 51. The method of claim 47, wherein etchant foretching the adhesion layer in the second etching operation containshydrogen fluoride (HF) when the adhesion layer is a titanium layer. 52.The method of claim 47, wherein etchant for etching the adhesion layerin the second etching operation contains phosphoric acid and acetic acidwhen the adhesion layer is an aluminum layer.
 53. The method of claim35, wherein material constituting the solder blocks does not wet thefirst under-ball metallic layer.
 54. A method of forming bumps over awafer having an active surface thereon, the method comprising the stepsof: forming a first under-ball metallic layer over the active surface ofthe wafer; forming a second under-ball metallic layer over the firstunder-ball metallic layer, conducting a first photolithography processto form a plurality of photoresist blocks over the second under-ballmetallic layer; conducting a first etching operation to remove thesecond under-ball metallic layer so that only the second under-ballmetallic layer underneath the photoresist blocks remains; afterconducting the first etching operation, removing the photoresist blocks;after removing the photoresist blocks, conducting a secondphotolithography process to form a photoresist layer over the firstunder-ball layer, wherein the photoresist layer has a plurality ofopenings that expose the second under-ball metallic layer; afterconducting the second photolithography process, conducting ametal-filling operation to fill a solder material into the openings ofthe photoresist layer, the solder material covering the secondunder-ball metallic layer; after conducting the metal-filling operation,conducting a first reflow operation to transform the solder materialinto a plurality of solder balls; after conducting the first reflowoperation, removing the photoresist layer; and after removing thephotoresist layer, conducting a second etching operation to remove aportion of the first under-ball metallic layer so that only the firstunder-ball metallic layer underneath the second under-ball metalliclayer remains.
 55. The method of claim 54, wherein after conducting thesecond etching operation, a second reflow operation is conducted. 56.The method of claim 54, wherein the step of forming the secondunder-ball metallic layer over the first under-ball metallic layerincludes the sub-steps of: forming a barrier layer over the firstunder-ball metallic layer; and forming a wettable layer over the barrierlayer.
 57. The method of claim 56, wherein material constituting thebarrier layer includes nickel-vanadium alloy.
 58. The method of claim57, wherein the first etching operation is carried out using an etchantcontaining sulfuric acid.
 59. The method of claim 58, wherein thebarrier layer having a thickness between 2000 Å and 4000 Å is etched forover 2 hours at room temperature using a sulfuric acid etchant having aconcentration between 1%˜98%.
 60. The method of claim 58, wherein thebarrier layer having a thickness between 2000 Å and 4000 Å is etched forover 2 hours at 80° C. using a sulfuric acid etchant having aconcentration between 1%˜98%.
 61. The method of claim 58, wherein thebarrier layer having a thickness between 2000 Å and to 4000 Å is etchedin the first etching operation by conducting an electromechanicaletching operation at room temperature for 20 to 110 seconds using acurrent density between 0.001˜0.02 A/cm² and sulfuric acid at 10%concentration.
 62. The method of claim 57, wherein the barrier layer isetched using diluted phosphoric acid in the first etching operation. 63.The method of claim 56, wherein material constituting the wettable layeris selected from a group consisting of copper, palladium and gold. 64.The method of claim 63, wherein the wettable layer is etched in thefirst etching operation using an etchant containing ammonium hydroxideand hydrogen peroxide if the wettable layer is a copper layer.
 65. Themethod of claim 63, wherein the wettable layer is etched in the firstetching operation using an etchant containing potassium sulfate (K₂SO₄)and glycerol if the wettable layer is a copper layer.
 66. The method ofclaim 54, wherein the first under-ball metallic layer includes anadhesion layer fabricated using a material selected from a groupconsisting of titanium, titanium-tungsten alloy, aluminum and chromium.67. The method of claim 66, wherein etchant for etching the adhesionlayer in the second etching operation contains hydrogen peroxide (H₂O₂),ethylene diamine tetraacetic (EDTA) and potassium sulfate (K₂SO₄) whenthe adhesion layer is a titanium-tungsten alloy layer.
 68. The method ofclaim 66, wherein etchant for etching the adhesion layer in the secondetching operation contains hydrogen chloride (HCl) when the adhesionlayer is a chromium layer.
 69. The method of claim 66, wherein etchantfor etching the adhesion layer in the second etching operation containsammonium hydroxide (NH₄OH) and hydrogen peroxide (H₂O₂) when theadhesion layer is a titanium layer.
 70. The method of claim 66, whereinetchant for etching the adhesion layer in the second etching operationcontains hydrogen fluoride (HF) when the adhesion layer is a titaniumlayer.
 71. The method of claim 66, wherein etchant for etching theadhesion layer in the second etching operation contains phosphoric acidand acetic acid when the adhesion layer is an aluminum layer.
 72. Themethod of claim 54, wherein material constituting the solder blocks doesnot wet the first under-ball metallic layer.
 73. A method of formingbumps over a wafer having an active surface thereon, the methodcomprising the steps of: forming a first under-ball metallic layer overthe active surface of the wafer; forming a second under-ball metalliclayer over the first under-ball metallic layer; conducting a firstphotolithography process to form a plurality of photoresist blocks overthe second under-ball metallic layer; conducting a first etchingoperation to remove the second under-ball metallic layer so that onlythe second under-ball metallic layer underneath the photoresist blocksremains; after conducting the first etching operation, removing thephotoresist blocks; after removing the photoresist blocks, conducting asecond photolithography process to form a photoresist layer over thefirst under-ball layer, wherein the photoresist layer has a plurality ofopenings that expose the second under-ball metallic layer; afterconducting the second photolithography process, conducting ametal-filling operation to fill a solder material into the openings ofthe photoresist layer, the solder material covering the secondunder-ball metallic layer; after conducting the metal-filling operation,removing the photoresist layer; after removing the photoresist layer,conducting a first reflow operation to transform the solder materialinto a plurality of solder balls; and after conducting the first reflowoperation, conducting a second etching operation to remove a portion ofthe first under-ball metallic layer so that only the first under-ballmetallic layer underneath the second under-ball metallic layer remains.74. The method of claim 73, wherein after conducting the second etchingoperation, a second reflow operation is conducted.
 75. The method ofclaim 73, wherein the first reflux operation is carried out after thephotoresist layer is removed.
 76. The method of claim 73, wherein thephotoresist layer is removed after the first reflux operation is earnedout.
 77. The method of claim 73, wherein the step of forming the secondunder-ball metallic layer over the first under-ball metallic layerincludes the sub-steps of: forming a barrier layer over the firstunder-ball metallic layer; and forming a wettable layer over the barrierlayer.
 78. The method of claim 77, wherein material constituting thebarrier layer includes nickel-vanadium alloy.
 79. The method of claim78, wherein the first etching operation is carried out using an etchantcontaining sulfuric acid.
 80. The method of claim 79, wherein thebarrier layer having a thickness between 2000 Å and to 4000 Å is etchedfor over 2 hours at room temperature using a sulfuric acid etchanthaving a concentration between 1%˜98%.
 81. The method of claim 79,wherein the barrier layer having a thickness between 2000 Å and to 4000Å is etched for over 2 hours at 80° C. using a sulfuric acid etchanthaving a concentration between 1%˜98%.
 82. The method of claim 79,wherein the barrier layer having a thickness between 2000 Å and to 4000Å is etched in the first etching operation by conducting anelectrochemical etching operation at room temperature for 20 to 110seconds using a current density between 0.001˜0.02 A/cm² and sulfuricacid at 10% concentration.
 83. The method of claim 78, wherein thebarrier layer is etched using diluted phosphoric acid in the firstetching operation.
 84. The method of claim 77, wherein materialconstituting the wettable layer is selected from a group consisting ofcopper, palladium and gold.
 85. The method of claim 84, wherein thewettable layer is etched in the first etching operation using an etchantcontaining ammonium hydroxide and hydrogen peroxide if the wettablelayer is a copper layer.
 86. The method of claim 84, wherein thewettable layer is etched in the first etching operation using an etchantcontaining potassium sulfate (K₂SO₄) and glycerol if the wettable layeris a copper layer.
 87. The method of claim 73, wherein the firstunder-ball metallic layer includes an adhesion layer fabricated using amaterial selected from a group consisting of titanium, titanium-tungstenalloy, aluminum and chromium.
 88. The method of claim 87, whereinetchant for etching the adhesion layer in the second etching operationcontains hydrogen peroxide (H₂O₂), ethylene diamine tetraacetic (EDTA)and potassium sulfate (K₂SO₄) when the adhesion layer is atitanium-tungsten alloy layer.
 89. The method of claim 87, whereinetchant for etching the adhesion layer in the second etching operationcontains hydrogen chloride (HCl) when the adhesion layer is a chromiumlayer.
 90. The method of claim 87, wherein etchant for etching theadhesion layer in the second etching operation contains ammoniumhydroxide (NH₄OH) and hydrogen peroxide (H₂O₂) when the adhesion layeris a titanium layer.
 91. The method of claim 87, wherein etchant foretching the adhesion layer in the second etching operation containshydrogen fluoride (HF) when the adhesion layer is a titanium layer. 92.The method of claim 87, wherein etchant for etching the adhesion layerin the second etching operation contains phosphoric acid and acetic acidwhen the adhesion layer is an aluminum layer.
 93. The method of claim73, wherein material constituting the solder blocks does not wet thefirst under-ball metallic layer.
 94. A method of forming bumps over theactive surface of a wafer, the method comprising the steps of: forming afirst under-ball metallic layer over the active surface of the wafer;forming a second under-ball metallic layer over the first under-ballmetallic layer; removing a portion of the second under-ball metalliclayer to expose the first under-ball metallic layer to the outside;after removing a portion of the second under-ball metallic layer,forming a solder material over the second under-ball metallic layer;after forming the solder material over the second under-ball metalliclayer, conducting a first reflow operation; and after conducting thefirst reflow operation, removing a portion of the first under-ballmetallic layer so that the first under-ball metallic layer underneaththe second under-ball metallic layer remains.
 95. The method of claim94, wherein after conducting the second etching operation, a secondreflow operation is conducted.
 96. The method of claim 94, wherein thestep of forming the second under-ball metallic layer over the firstunder-ball metallic layer includes the sub-steps of: forming a barrierlayer over the first under-ball metallic layer; and forming a wettablelayer over the barrier layer.
 97. The method of claim 96, whereinmaterial constituting the barrier layer includes nickel-vanadium alloy.98. The method of claim 97, wherein the first etching operation iscarried out using an etchant containing sulfuric acid.
 99. The method ofclaim 98, wherein the barrier layer having a thickness between 2000 Åand 4000 Å is etched for over 2 hours at room temperature using asulfuric acid etchant having a concentration between 1%˜98%.
 100. Themethod of claim 98, wherein the barrier layer having a thickness between2000 Å and 4000 Å is etched for over 2 hours at 80° C. using a sulfuricacid etchant having a concentration between 1%˜98%.
 101. The method ofclaim 98, wherein the barrier layer having a thickness between 2000 Åand 4000 Å is etched in the first etching operation by conducting anelectrochemical etching operation at room temperature for 20 to 110seconds using a current density between 0.001˜0.02 A/cm² and sulfuricacid at 10% concentration.
 102. The method of claim 97, wherein thebarrier layer is etched using diluted phosphoric aid in the firstetching operation.
 103. The method of claim 96, wherein materialconstituting the wettable layer is selected from a group consisting ofcopper, palladium and gold.
 104. The method of claim 103, wherein thewettable layer is etched in the first etching operation using an etchantcontaining ammonium hydroxide and hydrogen peroxide if the wettablelayer is a copper layer.
 105. The method of claim 103, wherein thewettable layer is etched in the first etching operation using an etchantcontaining potassium sulfate (K₂SO₄) and glycerol if the wettable layeris a copper layer.
 106. The method of claim 94, wherein the firstunder-ball metallic layer includes an adhesion layer fabricated using amaterial selected from a group consisting of titanium, titanium-tungstenalloy, aluminum and chromium.
 107. The method of claim 106, wherein theexposed first under-ball metallic layer is removed by etching andetchant for etching the adhesion layer contains hydrogen peroxide(H₂O₂), ethylene diamine tetraacetic (EDTA) and potassium sulfate(K₂SO₄) when the adhesion layer is a titanium-tungsten alloy layer. 108.The method of claim 106, wherein the exposed first under-ball metalliclayer is removed by etching and etchant for etching the adhesion layercontains hydrogen chloride (HCl) when the adhesion layer is a chromiumlayer.
 109. The method of claim 106, wherein the exposed firstunder-ball metallic layer is removed by etching and etchant for etchingthe adhesion layer contains ammonium hydroxide (NH₄OH) and hydrogenperoxide (H₂O₂) when the adhesion layer is a titanium layer.
 110. Themethod of claim 106, wherein the exposed first under-ball metallic layeris removed by etching and etchant for etching the adhesion layercontains hydrogen fluoride (HF) when the adhesion layer is a titaniumlayer.
 111. The method of claim 106, wherein the exposed firstunder-ball metallic layer is removed by etching and etchant for etchingthe adhesion layer contains phosphoric acid and acetic acid when theadhesion layer is an aluminum layer.
 112. The method of claim 94,wherein material constituting the solder blocks does not wet the firstunder-ball metallic layer.